U.S. patent number 6,123,890 [Application Number 08/892,757] was granted by the patent office on 2000-09-26 for methods for making pressure-sensitive adhesives having microstructured surfaces.
This patent grant is currently assigned to 3M Innovative Properties Company. Invention is credited to Gerald M. Benson, Robert K. Galkiewicz, Mieczyslaw H. Mazurek.
United States Patent |
6,123,890 |
Mazurek , et al. |
September 26, 2000 |
**Please see images for:
( Certificate of Correction ) ** |
Methods for making pressure-sensitive adhesives having
microstructured surfaces
Abstract
The invention provides pressure-sensitive adhesive (PSA) coated
articles, including tapes and transfer coatings, having
microstructured surfaces and methods of making pressure-sensitive
adhesive articles bearing such microstructured surfaces. The
performance properties of the pressure-sensitive adhesive articles
can be tailored by independently varying the microstructure and the
rheological properties of the pressure-sensitive adhesive.
Inventors: |
Mazurek; Mieczyslaw H.
(Roseville, MN), Galkiewicz; Robert K. (Roseville, MN),
Benson; Gerald M. (Woodbury, MN) |
Assignee: |
3M Innovative Properties
Company (St. Paul, MN)
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Family
ID: |
22513046 |
Appl.
No.: |
08/892,757 |
Filed: |
July 15, 1997 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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436021 |
May 5, 1995 |
5650215 |
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145423 |
Oct 29, 1993 |
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Current U.S.
Class: |
264/293; 264/299;
264/316 |
Current CPC
Class: |
B29C
43/28 (20130101); B29C 59/046 (20130101); C09J
7/38 (20180101); B29C 43/222 (20130101); C09J
7/10 (20180101); C09J 7/381 (20180101); C09J
2433/00 (20130101); Y10T 428/28 (20150115); Y10T
428/2883 (20150115); C09J 2301/204 (20200801); C09J
2453/00 (20130101); Y10T 428/24479 (20150115) |
Current International
Class: |
C09J
7/02 (20060101); B29C 043/00 () |
Field of
Search: |
;428/343,354
;264/39,1.31,1.32,1.34,293,299,316 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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0279579 |
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Sep 1988 |
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EP |
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3417746A1 |
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Nov 1985 |
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DE |
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3537433A1 |
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Oct 1986 |
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DE |
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3-243677 |
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Oct 1991 |
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JP |
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WO85/04602 |
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Oct 1985 |
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WO |
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WO94/00525 |
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Jan 1994 |
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WO |
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Other References
Sec. Ch. Wk. 9435, Derwent Pub. Ltd., London, GB; Class A81, AN
94-283515; and JP.A.6 212 131 (Sekisui Chem. Ind. Aug. 2, 1994).
.
International Search Report--PCT/US94/11563..
|
Primary Examiner: Davis; Jenna
Parent Case Text
This is a division of application Ser. No. 08/436,021, filed May 5,
1995, now U.S. Pat. No. 5,650,215, which is a continuation of
application Ser. No. 08/145,423, filed Oct. 29, 1993 (abandoned).
Claims
We claim:
1. A method of making a microstructured pressure-sensitive adhesive
tape comprising the steps of:
(a) providing a microstructured molding tool;
(b) embossing an adhesive layer of an adhesive tape comprising a
backing coated with a continuous layer of an embossable
pressure-sensitive adhesive with the microstructured molding tool,
wherein the pressure-sensitive adhesive layer is capable of
assuming the pattern of the microstructured molding tool and
retaining a microstructured surface upon removal from the
microstructured molding tool; and
(c) separating the microstructured molding tool and the adhesive
layer to form a microstructured pressure-sensitive adhesive
tape.
2. The method of claim 1, wherein the adhesive layer has a
thickness of about 10 .mu.m to about 250 .mu.m.
3. The method of claim 1, wherein the adhesive layer has a
thickness of about 25 .mu.m to about 150 .mu.m.
4. The method of claim 1, wherein the microstructured molding tool
is applied against the adhesive layer for about 0.1 second to about
5 minutes at a temperature of about 20.degree. C. to about
150.degree. C.
5. A method for making a microstructured pressure-sensitive
adhesive tape comprising the steps of:
(a) providing a microstructured backing having a pressure-sensitive
adhesive releasing microstructured side and a planar side having
less release character than the microstructured side;
(b) coating an embossable pressure-sensitive adhesive layer on the
planar side of the backing;
(c) contacting the surface of the pressure-sensitive adhesive layer
with the microstructured side of the backing to emboss the adhesive
layer; and
(d) separating the microstructured backing and the adhesive layer
to yield the microstructured pressure sensitive adhesive tape.
6. The method of claim 5, wherein the adhesive layer has a
thickness of about 10 microns to about 250 microns.
7. The method of claim 5, wherein the adhesive layer has a
thickness of about 25 microns to about 150 microns.
8. The method of claim 5, wherein the microstructured molding tool
is applied against the adhesive layer for about 0.1 seconds to
about 5 minutes at a temperature of about 29.degree. C. to about
150.degree. C.
9. A method of making a microstructured pressure-sensitive adhesive
transfer coating comprising the steps of:
(a) providing a microstructured molding tool;
(b) embossing an adhesive layer of an adhesive transfer coating
comprising a release liner coated with a continuous layer of an
embossable pressure-sensitive adhesive with the microstructured
molding tool, wherein the pressure-sensitive adhesive layer is
capable of assuming the pattern of the microstructured molding tool
and retaining a microstructured surface upon removal from the
microstructured molding tool; and
(c) separating the microstructured molding tool and the transfer
coating to form a microstructured pressure-sensitive adhesive
transfer coating.
10. The method of claim 9, wherein the adhesive layer has a
thickness of about 10 .mu.m to about 250 .mu.m.
11. The method of claim 9, wherein the adhesive layer has a
thickness of about 25 .mu.m to about 150 .mu.m.
12. The method of claim 9, wherein the microstructured molding tool
is applied against the adhesive layer for about 0.1 second to about
5 minutes at a temperature of about 20.degree. C. to about
150.degree. C.
Description
FIELD OF THE INVENTION
The invention relates to pressure-sensitive adhesive (PSA) coated
articles, including tapes and transfer coatings, having
microstructured surfaces and methods of making pressure-sensitive
adhesive articles bearing such microstructured surfaces. The
performance properties of the pressure-sensitive adhesive articles
can be tailored by independently varying the microstructure and the
rheological properties of the pressure-sensitive adhesive.
BACKGROUND OF THE INVENTION
Repositionable pressure-sensitive adhesives, adhesives which
predictably adhere to, yet remain repeatedly peelable from, a
variety of target substrates over a long period of time without
damaging or marring the substrate, have many commercial uses. For
example, masking tapes, removable labels or office notes,
protective films and medical tapes all must quickly adhere to
metal, paper, plastics and skin, respectively, but must also peel
smoothly away from these varied target substrates without leaving
behind any adhesive residue on or harming the surface of a
particular target substrate.
Several approaches have been explored in preparing and formulating
repositionable adhesives. One means for providing a repositionable
adhesive is through the reduction of the adhesive contact area and
can be accomplished by the deposition of a discontinuous or
patterned film on a backing. PCT International Appl. WO 85/04602
(Newing et al.) describes pressure-sensitive adhesive articles
comprising a plurality of discontinuous adhesive segments in a
pattern on at least a portion of at least one side of a carrier or
backing, covering from about ten to about thirty percent of the
total surface area of that carrier material. These segments have an
average height of from about 15 to about 35 microns and are about
50 to about 400 microns in width. The pressure-sensitive adhesive
coating used must have, according to American Society of Testing
Materials (ASTM) D-3330-81, a 180.degree. peel of from about 0.5 to
about 2.0 pound per inch (8.75 to 35 N/dm) when such adhesive is
coated and evaluated as specified by this standard. Finally, the
teachings of Newing et al. explicitly state that, ". . . running
together of the applied adhesive is to be avoided at all times . .
. ", as such a coalescence or coating continuity will hinder the
repositionability of these adhesives.
U.S. Pat. No. 4,587,152 (Gleichenhagen et al.) describes a
redetachable contact-adhesive sheet-like structure prepared by the
printing of a regular discontinuous pattern of calotte-shaped (cap
of sphere shaped) bonding sites up to 600 microns in diameter at
their base on a backing or carrier, such bonding sites comprising
an adhesive having a sufficiently high structural viscosity and
thixotropy to maintain their calotte shape. Gleichenhagen et al.
also teach that the adhesive properties of the claimed redetachable
sheet can be altered through the variation of the height, the
geometrical distribution, the frequency, and the basal diameter of
the calottes. It is further asserted that the adhesive properties
may be varied through controlling the viscoelastic properties of
the adhesive used (i.e., adhesive ranging, ". . . from very soft,
highly tacky and of low shearing strength to hard, slightly tacky
and of high shearing strength."). These rheological properties may
be further enhanced or controlled through crosslinking the adhesive
by heat or irradiation.
U.S. Pat. No. 5,194,299 (Fry) describes a repositionable
pressure-sensitive sheet material comprising a sheet material
bearing on one surface a discontinuous non-repetitive adhesive
coating covering about 10 to about 85 percent of the surface in the
form of individual adhesive islands. These islands, applied via
spray coating techniques, range from about 10 to about 150 microns
in height and from about 20 to about 500 microns in diameter at
their bases and are comprised of a pressure-sensitive adhesive
composition that, when coated continuously to a sheet material by
conventional means, would not remove cleanly from a paper substrate
or adherend. Fry also recognizes that the peel characteristics of
the claimed sheet material may be varied by controlling the
population density of the adhesive islands in the discontinuous
coatings and/or the inherent tackiness of the adhesive selected for
spray coating.
U.S. Pat. No. 4,889,234 (Sorenson et al.) discloses a discontinuous
patterned adhesive label structure in which the level of adhesion
is varied according to area of adhesive coverage on the label, the
pattern in which the adhesive is coated, and the full coverage
adhesive characteristics of the materials used. These variables may
be adjusted independently within a single label structure,
resulting in the capability to design differential peel forces at
specified portions of the label. Sorenson et al. teach the
criticality of selecting the adhesive material useful in the
claimed structures according to their 100% coverage (i.e.,
continuous coating) peel force, a quantity which ranges from
approximately 0.7 pound per inch (12.75 N/dm) for a solvent-type
removable adhesive to approximately 6 pounds per inch (105 N/dm)
for a solvent-based high strength adhesive in a 90.degree. peel
test from a stainless steel substrate. As a point of reference in
this disclosure, pressure-sensitive materials that are removable as
continuous, 100% coverage coatings, as specified by the Pressure
Sensitive Tape Council, have a peel force of about 2 pounds per
inch (35 N/dm) or less.
Yet another approach to providing a permanently repositionable
pressure-sensitive adhesive involves the use of crosslinking of a
continuous, planer coating to reduce the tack and control the
wetting or flow of the adhesive over the long term. U.S. Pat. No.
4,599,265 (Esmay) discloses a low tack, acrylate, removable
pressure-sensitive adhesive tape which maintains peelability from a
variety of ordinary target substrates. Esmay teaches that through
the photocrosslinking of the tape's adhesive layer and the use of
low levels of polar monomer (up to 3% by weight of a strongly polar
monomer, such as acrylic acid) along with alkyl acrylates having
side chains 8-12 carbons in length in the copolymeric adhesive, the
required balance of low tack, minimal adhesion buildup, and high
cohesive strength can be imparted to the removable adhesive.
U.S. Pat. No. 4,693,935 (Mazurek) discloses a continuous
pressure-sensitive adhesive coating composition comprising a
copolymer having a vinyl polymeric backbone having grafted thereto
polysiloxane moieties. An exposed surface of the PSA coating is
initially positionable on a target substrate to which it will be
adhered to but, once adhered, builds adhesion to form a strong
bond.
European Patent Appl. 279,579 B1 (Tanuma et al.) describes
pressure-sensitive adhesive sheets comprising, in one embodiment, a
continuous adhesive layer "having a macroscopically non-uniform
adhesion face". These pressure-sensitive adhesive constructions,
formulated to exhibit both initial and long term repositionability
on a variety of target substrates, attain these removable
characteristics through a combination of the partial contact
between adhesive layer and adherend caused by this uneven adhesive
layer and through the introduction of a crosslinking structure to
the adhesive to limit the adhesion build up resulting from the
fluidity or flow of the adhesive over the long term. The uneven
adhesive layer, according to the application, is imparted through a
variety of pressing, molding, and embossing methods.
SUMMARY OF THE INVENTION
A need thus exists for a continuously coated, unfilled,
microstructured pressure-sensitive adhesive article which exhibits
initial repositionability when adhered to a variety of target
substrates and, through the independent variation and selection of
microstructured pattern and the chemical nature and rheological
properties of the microstructured pressure-sensitive adhesive,
displays reduced, constant or increased long-term adhesion as
required by the intended application.
A need further exists for methods of preparing such microstructured
pressure-sensitive adhesive articles.
The present invention relates to an article, including adhesive
tapes and transfer coatings, bearing a continuous
pressure-sensitive adhesive layer having a microstructured surface
wherein the microstructured surface comprises a series of features
and wherein the lateral aspect ratio of the features range from
about 0.1 to about 10. At least two of the feature dimensions
(height, width and length) must be microscopic. All three of the
feature dimensions (height, width, length) may be microscopic. The
microstructured patterned adhesive exhibits initial
repositionability when adhered to a variety of target substrates
and, through the independent variation and selection of
microstructured pattern and the chemical nature and rheological
properties of the microstructured pressure-sensitive adhesive,
displays reduced, constant, or increased long-term adhesion as
required by the intended application.
Another aspect of the present invention relates to a first method
of making a microstructured pressure-sensitive adhesive tape
comprising the steps of:
(a) providing a microstructured molding tool;
(b) embossing an adhesive layer of an adhesive tape comprising a
backing coated with a continuous layer of an embossable
pressure-sensitive adhesive with the microstructured molding tool,
wherein the pressure-sensitive adhesive layer is capable of
assuming the pattern of the microstructured molding tool and
retaining a microstructured surface upon removal from the
microstructured molding tool; and
(c) separating the microstructured molding tool from the adhesive
layer to form a microstructured pressure-sensitive adhesive
tape.
Another aspect of the present invention relates to a second method
of making a microstructured pressure-sensitive adhesive tape
comprising the steps of:
(a) providing a microstructured molding tool;
(b) coating a pressure-sensitive adhesive layer against the
microstructured molding tool, wherein the pressure-sensitive
adhesive layer is capable of assuming the pattern of the
microstructured molding tool and retaining the microstructured
pattern upon removal from the microstructured molding tool;
(c) applying a backing to the surface of the pressure-sensitive
adhesive layer which is in contact with the microstructured molding
tool; and
(d) separating the microstructured molding tool and the adhesive
layer to form a microstructured pressure-sensitive adhesive
tape.
Another aspect of the present invention relates to a third method
for making a microstructured pressure-sensitive adhesive tape
comprising the steps of:
(a) providing a microstructured backing having a pressure-sensitive
adhesive releasing microstructured side and a planar side having
less release character than the microstructured side;
(b) coating a pressure-sensitive adhesive layer on the
microstructured side of the backing;
(c) adhering the surface of the pressure-sensitive adhesive layer
which is in contact with the microstructured backing to the planar
side of the
microstructured backing; and
(d) removing the microstructured side of the backing from the
microstructured surface of the adhesive layer to form a
microstructured pressure-sensitive adhesive tape.
Another aspect of the present invention relates to a fourth method
for making a microstructured pressure-sensitive adhesive tape
comprising the steps of:
(a) providing a microstructured backing having a pressure-sensitive
adhesive releasing microstructured side and a planar side having
less release character than the microstructured side;
(b) coating an embossable pressure-sensitive adhesive layer on the
planar side of the backing;
(c) contacting the surface of the pressure-sensitive adhesive layer
which is in contact with the microstructured backing with the
microstructured side of the backing to emboss the adhesive layer;
and
(d) separating the microstructured backing and the adhesive layer
to yield a microstructured pressure sensitive adhesive tape.
Another aspect of the present invention relates to a first method
of making a microstructured pressure-sensitive adhesive transfer
coating comprising the steps of:
(a) providing a microstructured molding tool;
(b) embossing an adhesive layer of an adhesive transfer coating
comprising a release liner coated with a continuous layer of an
embossable pressure-sensitive adhesive with the microstructured
molding tool, wherein the pressure-sensitive adhesive layer is
capable of assuming the pattern of the microstructured molding tool
and retaining a microstructured surface upon removal from the
microstructured molding tool; and
(c) separating the microstructured molding tool and the transfer
coating to form a microstructured pressure-sensitive adhesive
transfer coating.
Another aspect of the present invention relates to a second method
of making a microstructured pressure-sensitive adhesive transfer
coating comprising the steps of:
(a) providing a microstructured molding tool;
(b) coating a pressure-sensitive adhesive layer against the
microstructured molding tool, wherein the pressure-sensitive
adhesive layer is capable of assuming the pattern of the
microstructured molding tool and retaining the microstructured
pattern upon removal of the microstructured molding tool;
(c) applying a release liner to the surface of the
pressure-sensitive adhesive layer which is in contact with the
microstructured molding tool; and
(d) separating the microstructured molding tool and the adhesive
layer to form a microstructured pressure-sensitive adhesive
transfer coating.
Another aspect of the present invention relates to a third method
of making a microstructured pressure-sensitive adhesive transfer
coating comprising the steps of:
(a) providing a first release liner coated with a continuous layer
of an embossable pressure-sensitive adhesive; and
(b) embossing the surface of the pressure-sensitive adhesive layer
which is in contact with the first release liner with a
microstructured second release liner to form a microstructured
pressure-sensitive adhesive transfer coating.
Another aspect of the present invention relates to a fourth method
of making a microstructured pressure-sensitive adhesive transfer
coating comprising the steps of:
(a) providing a microstructured liner having a microstructured side
and a planar side, both sides having release characteristics;
(b) coating a pressure-sensitive adhesive layer on the
microstructured side of the liner;
(c) adhering the surface of the pressure-sensitive adhesive layer
which is not in contact with the microstructured liner to the
planar side of the microstructured liner; and
(d) removing both the microstructured side and planar side of the
liner from the adhesive layer to form a microstructure
pressure-sensitive adhesive transfer coating.
Definitions
The following terms are used herein.
As used herein, the term "microscopic" refers to features of small
enough dimension so as to require an optic aid to the naked eye
when viewed from any plane of view to determine its shape. One
criterion is found in Modern Optic Engineering by W. J. Smith,
McGraw-Hill, 1966, pages 104-105 whereby visual acuity, ". . . is
defined and measured in terms of the angular size of the smallest
character that can be recognized." Normal visual acuity is
considered to be when the smallest recognizable letter subtends an
angular height of 5 minutes of arc on the retina. At at typical
working distance of 250 mm (10 inches), this yields a lateral
dimension of 0.36 mm (0.0145 inch) for this object.
As used herein, the term "microstructure" means the configuration
of features wherein at least 2 dimensions of the features are
microscopic. The topical and/or cross-sectional view of the
features must be microscopic. The function of the pressure
sensitive adhesive article is critically dependent on the form of
the microstructure, which may consist of positive and negative
features.
As used herein, the term "positive features" means features
projecting out of the body of the microstructured molding tool,
microstructured liner, microstructured backing, or microstructured
pressure-sensitive adhesive layer.
As used herein, the term "negative features" means features
projecting into the body of the microstructured molding tool,
microstructured liner, microstructured backing, or microstructured
pressure-sensitive adhesive layer.
As used herein, the term "embossable" refers to the ability of a
pressure-sensitive adhesive layer to have part of its surface
raised in relief, especially by mechanical means.
As used herein, the term "wetting" means spreading out over and
intimately contacting a surface.
As used herein, the term "dewetting" means contracting from
intimate contact with a surface.
As used herein, the term "repositionable adhesives" refers to those
adhesives which upon application to a specific target substrate can
be removed without causing damage to the substrate and without
leaving residue on the substrate and without causing damage to the
backing or liner over a range of peel forces.
As used herein, the term "permanently repositionable adhesives"
refers to repositionable adhesives for which the of adhesion to a
given target substrate does not change substantially with time
under application conditions.
As used herein, the term "temporarily repositionable adhesives"
refers to those initially repositionable adhesives which build in
adhesion with time, pressure or temperature such that they are no
longer repositionable.
As used herein, the term "self-debonding adhesives" refers to
adhesives which show initial adhesion controlled by the conditions
of application (pressure) and a decrease of the adhesion level with
time.
As used herein, the term "release liner", used interchangeably with
the term "liner", refers to a thin flexible sheet which after being
placed in intimate contact with pressure-sensitive adhesive surface
may be subsequently removed without damaging the adhesive
coating.
As used herein, the term "microstructured liner" refers to a liner
with a microstructured surface.
As used herein, the term "backing" refers to a thin, flexible sheet
which, after being placed in intimate contact with
pressure-sensitive adhesive can not be subsequently removed without
damaging adhesive coating.
As used herein, the term "microstructured backing" refers to a
backing with a microstructured surface.
As used herein, the term "target substrate" refers to a surface to
which the pressure-sensitive adhesive coating is applied for an
intended purpose.
As used herein, the term "tape" refers to a pressure-sensitive
adhesive coating applied to a backing.
As used herein, the term "transfer coating" refers to a layer of
pressure-sensitive adhesive, which is not supported by a
backing.
DRAWINGS
FIG. 1 illustrates both a first method of making a
pressure-sensitive adhesive tape of the invention and a first
method of making a pressure-sensitive adhesive transfer coating of
the present invention.
FIG. 2 illustrates both a second method of making a
pressure-sensitive adhesive tape of the invention and a second
method of making a pressure-sensitive adhesive transfer coating of
the present invention.
FIGS. 3a-3c illustrate both a third method of making a
pressure-sensitive adhesive tape of the invention and a fourth
method of making a pressure-sensitive adhesive transfer coating of
the present invention.
FIGS. 4a-4c illustrate a fourth method of making a
pressure-sensitive adhesive tape of the invention.
FIGS. 5a-5c illustrate a third method of making a
pressure-sensitive adhesive transfer coating of the invention.
FIG. 6 illustrates a scanning electron micrograph of a convex
hemispheric microstructured surface.
FIG. 7 illustrates a top plan view of a microstructured molding
tool having cube corners of positive features.
FIG. 8 illustrates a cross-sectional view as indicated by FIG. 7
and illustrates a microstructured molding tool having cube corners
of positive features.
FIG. 9 illustrates a cross-sectional view of a microstructured
molding tool having cube corners of negative features.
DETAILED DESCRIPTION OF THE INVENTION
We have discovered that the appropriate combination of
microstructured pattern and rheological characteristics of the
pressure-sensitive adhesive making up the pressure-sensitive
adhesive layers of the tapes and transfer coatings of the present
invention provides a means of controlling repositionability
characteristics, thus allowing one to make pressure-sensitive
adhesive articles which are temporarily repositionable, permanently
repositionable, or self-debonding.
In the case of temporarily repositionable pressure-sensitive
adhesive tapes and transfer coatings, it is desirable that the
microstructured surface of the adhesive article retain its shape
until pressure is applied to establish firm contact of a
pressure-sensitive coating with a target substrate, or the adhesive
layer flows and makes continuous contact with the target substrate
due to exposure to heat and/or through inherent surface wetting and
rheological properties of the adhesive composition.
In the case of permanently repositionable pressure-sensitive
adhesive tapes and transfer coatings, it is desirable that the
microstructured surface of the adhesive article retain its shape
(i.e., preserves discontinuity of the contact areas with a target
substrate) indefinitely at the temperature and pressure range
required for a specific application.
In the case of self-debonding pressure-sensitive adhesive tapes and
transfer coatings, it is desirable that the microstructured surface
of the adhesive article retains its shape indefinitely at the
application and the use temperature and pressure and the elasticity
recovery forces in the adhesive layer are able to bring about the
controlled shape recovery after the adhesive has been
pressure-applied to a target substrate.
In terms of pressure-sensitive adhesive compositions, different
sets of requirements exist for hot-melt, radiation curable, solvent
and water-based adhesives. The only general requirements are that
the pressure-sensitive adhesives, as coated, must be able to:
assume a microstructured surface as imparted from a microstructured
molding tool, backing or liner; retain this surface during the
separation of the microstructured molding tool, backing or liner
from the pressure-sensitive adhesive; and, retain a microstructured
surface as long as required by a specific application.
Methods of Making Microstructured Tapes
The first method of the present invention illustrated in FIG. 1
involves the use of a microstructured molding tool [4] to emboss a
continuous layer of a pressure-sensitive adhesive [2] having a
planer surface [2a] coated on a backing [1]. The thickness of the
adhesive layer [2] which is embossed by such microstructured
molding tool [4] can vary depending upon the requirement of the
final application. The adhesive layer [2] must be thick enough such
that after embossing a continuous structured adhesive [2b] must
exist. Typically, the adhesive layer [2] is coated at a thickness
of about 10 .mu.m to about 250 .mu.m, preferably about 25 to about
150 .mu.m. The microstructured molding tool [4] is applied against
the adhesive surface [2a] for a sufficient time and at a sufficient
temperature and pressure to impart the desired features to provide
a continuous adhesive layer having a microstructured surface [2b]
(typically about 0.1 second to about 5 minutes at a temperature of
about 20.degree. C. to about 150.degree. C.) depending on the
adhesive and microstructure surface desired. The pressure is then
discontinued and the sample allowed to cool. The mold is
subsequently separated from the adhesive layer yielding a
microstructured pressure-sensitive adhesive surface [2b] which
substantially replicates the shaping and pattern of the particular
microstructured molding tool [4].
The second method illustrated by FIG. 2 involves coating or
extruding a layer of pressure-sensitive adhesive [5] onto a
microstructured molding tool [4]. The surface of the pressure
sensitive adhesive layer which does not come into contact with the
microstructured molding tool [5a] (i.e., the exposed surface) is
then transferred to a substrate [1], which in this case is a
backing, to form a microstructured pressure-sensitive adhesive
tape. To ensure a clean separation of the adhesive layer [5] from
the microstructure molding tool [4], the adhesion of the adhesive
surface [5a] to the backing must be greater than the adhesion of
the microstructured adhesive surface [5b] to the molding tool
[4].
The third method illustrated by FIGS. 3a-3c involves the coating or
extruding of a pressure-sensitive adhesive layer [6] from an
adhesive reservoir [7] onto a microstructured surface [8a] of a
backing [8] which has been previously microstructured on one of its
major surfaces. Referring to FIG. 3a, the microstructured surface
[8a] of the backing [8] must be capable of releasing the
microstructured surface [6b] of the adhesive layer, either via
treatment with a low surface energy release coating or through the
inherent release characteristics of the microstructured surface
[8a] of the backing. Referring to FIG. 3b, the exposed surface of
the pressure-sensitive adhesive layer [6a] is then adhered to the
non-microstructured (i.e., planar) side [8b] of the backing [8],
transferring the adhesive layer [6] to the planar surface [8b] of
the backing and revealing the microstructured adhesive surface [6b]
as the adhesive layer [6] is separated from the microstructured
surface [8a] of the backing (FIG. 3c). This planar backing surface
[8b] must have a higher tendency or affinity toward bonding with
the planar surface of the adhesive [6a] than the microstructured
backing surface [8a] to the microstructured adhesive surface [6b]
to facilitate a clean removal and replication of the microstructure
in the surface of the microstructured pressure-sensitive adhesive
tape. A preferred means for obtaining the final adhesive article
according to this method is to wind or roll the initially coated
structure upon itself in a convoluted manner (FIG. 3b). The
combination of the compressive forces in this tape roll and the
greater adhesion of the adhesive layer [6] to the planar surface
[8b] of the backing [8] causes, upon unwinding the tape (FIG. 3c),
the microstructured adhesive layer [6] to cleanly transfer to the
planar side [8b] of the backing [8].
The fourth method illustrated by FIGS. 4a-4c involves coating an
adhesive layer [9] on a similarly prepared or procured
microstructured backing [10] as described supra in the third method
of the present invention (FIG. 4a).
The adhesive layer [9] in this method, however, is coated from the
adhesive reservoir [11] on the planar backing surface [10a] and
placed into contact with the microstructured surface [10b] of the
backing. Referring to FIG. 4b, the pressure-sensitive adhesive
layer [9] is embossed through this contact with the releasable
microstructured backing surface [10b] and, thus, must be a
composition capable of flowing under the thermal and compressive
forces present in the process and storage conditions to replicate
the features of the microstructure under the conditions of this
contact. Again, a preferred means for embossing the adhesive layer
[9] of a microstructure pressure-sensitive adhesive tape made by
this fourth method is to wind the coated backing upon itself to
form a roll (FIG. 4b). Referring to FIG. 4c, the microstructured
surface [9b] the adhesive is then separated from the
microstructured surface [10b] of the backing, yielding an exposed
microstructured adhesive surface [9b].
Methods of Making Microstructured Transfer Coatings
A microstructured adhesive transfer coating may be prepared through
the substitution of backings in the adhesive tapes of the first and
second methods with release liners. Thus in a first method of
making an adhesive transfer coating and an adaptation of the first
method of making a microstructured adhesive tape, and as
illustrated in FIG. 1, a pressure-sensitive adhesive layer [2] is
first coated onto a substrate [1], which is a liner rather than a
backing, and then the surface of the adhesive layer [2a] is
embossed with and released from a microstructured molding tool [4]
to form a microstructured adhesive surface [2b].
In a second method for making a microstructured transfer coating,
and as illustrated in FIG. 2, the second method for making a
microstructured tape is merely altered to produce a microstructured
adhesive transfer coating by applying a substrate [1], which in
this case is a release liner, instead of a backing, to the exposed
surface [5a] of the pressure-sensitive adhesive layer [5] which has
been coated onto a microstructured molding tool [4].
Both of these methods result in a transfer coating in which the
release liner [1] is adhered to the non-embossed surfaces of the
adhesive layers ([2] and [5]) and the microstructured surface of
the adhesive layers are exposed. Another release liner can then be
placed over the microstructured surfaces ([2b] or [5b]) of the
adhesive layers ([2] or [5]) provided that the release surfaces do
not encourage wetting by the adhesive and/or the rheological nature
of the adhesive is such that it retains the microstructure without
substantial deformation.
Should a transfer coating be desired in which the microstructured
surface of the pressure-sensitive adhesive layer requires a release
liner to preserve or protect the microstructured pattern, then the
above methods can be further modified by replacing the
microstructured molding tools used to impart a pattern into the
adhesive with microstructured liners. Thus, instead of releasing
the mold from the final articles as in the tape and transfer
coatings described above, these methods for preparing transfer
coatings result in articles which further comprise the mold used to
impart the structure into the adhesive layer.
In a third method for making a microstructured pressure-sensitive
adhesive transfer coating, and as illustrated in FIG. 5a, a
microstructured liner [8] is employed to emboss a microstructure
onto a pressure-sensitive adhesive layer [12] coated from reservoir
[11] which is being carried by another release liner [13] in an
adaptation of the first method of making such coatings. The
microstructured surface [8a] of the liner is embossed into the
adhesive layer [12] by rollers, platen, or other means of
compressing the liner's microstructured surface [8a] against the
adhesive layer surface [12a]. If the release liners utilized are
flexible, a microstructured surface may be imparted as illustrated
in FIG. 5b by winding the adhesive layer [12], between the two
release liners [8] and [13] into a roll. Similar to the convoluted
tape roll embodiments describe supra, the intimate contact and
compressive forces exerted between the adhesive layer [12] and the
microstructured liner [8] are sufficient to impart a
microstructured surface into these adhesive layers. As the roll is
unwound as demonstrated by FIG. 5c, either release liner may be
removed to expose the desired surface (either the planer surface
[12b] or the microstructured surface [12c]) of the adhesive layer
depending on the application and requirements of the
pressure-sensitive adhesive transfer coating.
As illustrated in FIGS. 3a-3c, by coating a pressure-sensitive
adhesive layer [6] onto the microstructured surface [8a] of a
microstructured release liner [8] rather than a microstructured
molding tool, a fourth method for making a microstructured transfer
coating may be similarly adapted. If both surfaces [8a] and [8b] of
the microstructured liner [8] have release characteristics, then
the coated liner of FIG. 3a can be wound upon itself to form a roll
as illustrated in FIG. 3b. As illustrated in FIG. 3c, as the roll
is unwound, the microstructured pressure-sensitive adhesive
transfer coating releases at surface [6a] and/or [6b] and can be
applied to a target substrate. Alternatively, a second release
liner may be applied to the exposed planar surface [6a] of the
adhesive layer [6] prior to winding.
Pressure-sensitive adhesive transfer coatings having the same or
different microstructures on both surfaces of the adhesive layer
can also be formed by alteration or combinations of these methods.
For example, the first method may be altered by coating an adhesive
layer onto a microstructured liner, followed by the embossing the
exposed surface of the adhesive layer by a microstructured molding
tool. In another embodiment, such transfer coatings can be prepared
from an embossable adhesive layer which is microstructured by
simultaneously or sequentially embossing both surfaces of the layer
using microstructured molding tools or liners having the same or
different microstructured patterns. Such articles can be useful in
attaching or joining target substrates where the bond to each
substrate must be initially repositionable. The long-term adhesion
to each target substrate can then be set depending on the
microstructures selected and the rheological properties of the
adhesive layer against each target substrate.
Microstructured Molding Tools
A microstructured molding tool is an implement for imparting a
structure or finish to a pressure-sensitive adhesive coating and
which may be continuously reused in the process. Microstructured
molding tools can be in the form of a planar stamping press, a
flexible or inflexible belt, or a roller. Furthermore,
microstructured molding tools are generally considered to be tools
from which the microstructured adhesive pattern is generated by
embossing, coating, casting, or platen pressing and do not become
part of the finished microstructured adhesive article.
A broad range of methods are known to those skilled in this art for
generating microstructured molding tools. Examples of these methods
include but are not limited to photolithography, etching, discharge
machining, ion milling, micromachining, and electroforming.
Microstructured molding tools can also be prepared by replicating
various microstructured surfaces, including irregular shapes and
patterns, with a moldable material such as those selected from the
group consisting of crosslinkable liquid silicone rubber, radiation
curable urethanes, etc. or replicating various microstructures by
electroforming to generate a negative or positive replica
intermediate or final embossing tool mold. Also, microstructured
molds having random and irregular shapes and patterns can be
generated by chemical etching, sandblasting, shot peening or
sinking discrete structured particles in a moldable material.
Additionally any of the microstructured molding tools can be
altered or modified according to the procedure taught in Benson
U.S. Pat. No. 5,122,902 assigned to the assignee of the present
case incorporated by reference herein. Finally, the microstructured
molding tool must be capable of separating cleanly from the
pressure-sensitive adhesive layer.
Microstructured Backings and Liners
Typically the microstructured backings and liners are made from
materials selected from the group consisting of embossable or
moldable materials having sufficient structural integrity to enable
them to withstand the process of conveying the microstructure to
the adhesive and be cleanly removed from the microstructured
adhesive layer. Preferred materials which the microstructured liner
may comprise include but are not limited to those selected from the
group consisting of plastics such as polyethylene, polypropylene,
polyesters, cellulose acetate, polyvinylchloride, and
polyvinylidene fluoride, as well as paper or other substrates
coated or laminated with such plastics. These embossable coated
papers or thermoplastic films are often siliconized or otherwise
treated to impart improved release characteristics. As noted in the
discussions of methods for making the tapes and transfer coatings
of the present invention, depending on the method employed and the
requirements of the final article, one or both sides of these
backings or liners must have release characteristics.
Microstructured liners and backings are available commercially from
a number of sources. Specific examples of such include but are not
limited to microstructured polyethylene and polypropylene coated
paper liners of various densities such as those commercially
available from P/S Substrates, Inc., Schoeller Technical Papers,
Inc., and P.W.A. Kunstoff GMBH.
Features of Microstructured Surfaces
The microstructured molding tools, liners, backings, and,
ultimately, the microstructured pressure-sensitive adhesive tapes
and transfer coatings of the present invention have a multiplicity
of projection features. The term "projection features" as used
herein covers both negative and positive configurations providing
microstructured adhesives with positive and negative
configurations, respectively. These features are commonly referred
to as negative or positive structures by those who are familiar in
the art of microstructured technology. Each feature should or
typically have a height of about 2.5 micrometers (0.0001") to about
375 micrometers (0.015"), preferably about 25 micrometers (0.001")
to about 250 micrometers (0.010"), and most preferably about 25
micrometers (0.001") to about 125 micrometers (0.005") for reasons
of minimizing thickness of the adhesive, increasing the density of
the microstructured adhesive pattern sizes for symmetric patterns,
and controlling the adhesion levels.
The shape of the features in the microstructured molding tool,
backing or liner and the microstructured pressure-sensitive
adhesive articles prepared therefrom can vary. Examples of feature
shapes include but are not limited to those selected from the group
consisting of hemispheres, prisms (such as square prisms,
rectangular prisms, cylindrical prisms and other similar polygonal
features), pyramids, ellipses, and grooves. Positive or negative
features can be employed, i.e. convex hemispheres or concave
hemispheres, respectively. The preferred shapes include those
selected from the group consisting of hemispheres, pyramids (such
as cube corners, tetrahedra, etc.), and "V" grooves, for reasons of
pattern density, adhesive performance, and readily available
methodology of the microstructured pattern generation or
development. Although the exemplified features are non-truncated in
nature, it is believed that truncated features will also be
suitable in the articles of the present invention. The features of
the microstructured liner may be systematically or randomly
generated.
The limits of lateral dimensions of the features can be described
by use of the lateral aspect ratio (LAR) which is defined as the
ratio of the greatest microscopic dimension of the feature parallel
to the plane of the continuous layer of adhesive to either the
height of a positive feature or depth of a negative feature. Too
large a LAR leads to a short squat feature that would not provide
the advantages of microstructuring. Too small a LAR would lead to a
tall narrow feature which would not stand upright due to the low
flexural modulus of the pressure-sensitive adhesive (and therefore
low flexural rigidity of the feature). That is, typical
pressure-sensitive adhesive rheological properties will not support
too small a LAR whereas too large a LAR approaches the realm of
conventional pressure-sensitive adhesive tapes. Typical limits of
the LAR would be about 0.1 to about 10, with most preferred limits
of about 0.2 and about 5.
The nearest neighbor distance between features can be specified
with a spacing aspect ratio (SAR) given by the ratio of
center-to-center nearest neighbor distance to feature the greatest
lateral microscopic dimension as defined for the LAR. The minimum
value the SAR can assume is 1 which corresponds to the sides of
features touching. This value is most useful for features such as
hemispheres and pyramids which taper towards the top of the
feature. For non-tapering and reverse-tapering features such as
rods, square prisms, rectangular prisms, inverted cones,
hemispheres, and pyramids, the SAR should be greater than 1 so that
the perimeters of the top of the features do not touch and so form
a new planar surface. A typical upper limit for the SAR would be
1.9 and a more desirable upper limit would be 1.5. A most preferred
upper limit would be 1.1.
If the SAR is too great, positive features may not be able to
support the remainder of the PSA above the surface. This leads to
more extensive areas of contact between the PSA and target
substrate than would be calculated using just the feature
dimensions. That is, the adhesive comprising the "lands" between
positive features would sag or flex and touch the target surface.
The adhesive comprising the surface between negative features would
comprise such a large continuous planar surface at a high SAR so as
to render the features irrelevant for modification of peel forces.
In either case of positive or negative features, carried to an
extreme, a large SAR would lead to essentially a planar
adhesive.
A pattern with asymmetry could be defined by multiple SARs. In the
case of multiple SARs, all SARs should obey the limits listed
above.
For example, one SAR might be concerned with both the feature width
and nearest neighbor distance in the machine direction of the tape
or transfer coating; this could be termed SAR.sub.md. In a similar
manner one could define a cross-direction ratio, SAR.sub.cd, which
is concerned with both the feature width and nearest neighbor
distance in the cross (or transverse) direction of the tape or
transfer coating. For patterns with one lateral macroscopic
dimension, such as parallel V-grooves running in the machine
direction, both the width and the nearest neighbor distance go to
infinity, leading to an SAR.sub.md of 1.
The percentage of the surface area of a coating or liner which
comprises features as opposed to flat surface is given
approximately by:
For one dimensional features such as V-grooves, where SAR.sub.md
=1, SAR.sub.cd values of 1, 1.1, 1.5, and 1.9 produce percent
coverages of 100, 91, 67 and 53%, respectively. For symmetric
two-dimensional features such as hemispheres, pyramids, etc. spaced
apart equally in the machine- and cross-directions, we have
SAR.sub.md equal to SAR.sub.cd and SAR values of 1, 1.1, 1.5 and
1.9 produce percent coverages of 100, 83, 44, and 28%,
respectively. For the general asymmetric case where SAR.sub.md does
not equal SAR.sub.cd, the percent coverage values have to be
calculated using the above equation on a case by case basis.
However, the limitations on the values of the SAR in any one
direction as described earlier still apply. It should be noted that
the above equation is a guide and may not apply accurately to very
non-uniform patterns such as re-entrant features or random features
or mixtures of different size and shape features on the same
surface.
General Properties of Microstructured Pressure-Sensitive
Adhesives
When a smooth (planar) adhesive undergoes changes in adhesion to a
planar target substrate over time there are at least two possible
effects which may be the cause. The first of these effects is a
change of the chemical affinity of the adhesive surface towards the
target substrate. This may take place through the movement in the
polymeric chain either towards or away from the interface. The
second of these effects is a flow of the polymer on the scale of
nanometers to accommodate to the surface irregularities of the
substrate; this scale of surface wetting enhancement cannot
currently be detected optically.
The time dependency of the adhesion of the microstructured
adhesives can
possibly be attributed to a third and possible fourth effects which
work along with the two above effects. This third effect is the
change of the structure shape in response to the balance between
the surface affinity of the two materials and the elastic recovery
forces in the adhesive. This shape change will take place on a size
scale of the order of micrometers for the microstructures of the
present invention. The fourth effect is the trapping of air within
negative features sealed to the target substrate. This trapped air
works to keep the adhesive surface away from the target substrate
surface and so to frustrate surface wetting. It should be noted
that this same mechanism will frustrate adhesion in planar
adhesives which are carelessly rolled onto a smooth substrate so as
to trap air in pockets.
The selection of a positive or negative projection configuration
for a particular projection feature will affect the peel adhesion
characteristics of the resultant microstructured adhesive surface.
Certain positive projection features such as positive hemispheres
and positive pyramids contact a target substrate such that the
initial contact area (the tip of the pyramid for example) has a
smaller cross-section than the remainder of the projection feature.
In those cases the choice of the adhesive will be critical in
determining the long term characteristics of the microstructured
adhesive coating. If a pressure sensitive adhesive is selected with
properties such that the adhesion forces between the
microstructured adhesive coating and the target substrate are
stronger than the elastomeric recovery forces of the portion of the
microstructured adhesive deformed upon application of the coating
to the substrate, wetting can increase over time leading to a
corresponding increase of peel adhesion level. Alternatively, if a
pressure sensitive adhesive is selected with properties such that
the adhesion forces between the adhesive portion in contact with
the target substrate and the substrate are counterbalanced by the
elastomeric recovery forces of the microstructured portion of the
adhesive coating, wetting will remain nearly constant and the
corresponding peel adhesion level will not change drastically over
time.
Finally, if a pressure sensitive adhesive is selected with
properties such that the adhesion forces between the
microstructured adhesive coating and the substrate are weaker than
the elastomeric recovery forces of the portion of the
microstructured adhesive deformed upon application of the coating
to the substrate, wetting can decrease over time leading to a
corresponding decrease of peel adhesion level.
Non-tapering positive projections, such as cylindrical projections
or cube projections (in which an entire face of the cube contacts
the substrate), would not be expected to build in adhesion over
time since the lateral dimension (cross-section) at the point of
contact, and the base, as well as the entire length of the
projection is the same.
Negative hemisphere or pyramid projections would not, as mentioned
previously, provide microstructured surfaces that build
significantly in peel adhesion over time. These negative projection
features have an adhesive distribution such that the adhesive in
the projection feature which does not initially contact the
substrate does not completely wet the target substrate over time;
thus, peel adhesion does not build appreciably over time.
Pressure-Sensitive Adhesives
Useful pressure-sensitive adhesives for the purposes of the present
invention include those which are capable of retaining a
microstructured surface after being embossed with a microstructured
molding tool, backing or liner or after being coated on a
microstructured molding tool, backing or liner from which it is
subsequently removed. The particular pressure-sensitive adhesive
used depends upon the microstructuring method employed in producing
the microstructured pressure-sensitive adhesive article and the
short and long term peel characteristics required in the final
product. Finally, useful microstructured pressure-sensitive
adhesive layers should be capable of retaining their
microstructured surfaces for a time sufficient to allow for
transport, storage, and handling before the ultimate utilization of
the adhesive tape or transfer coating.
When an embossing process is used, a microstructure is imparted
upon the continuous adhesive surface as defined by the pattern set
by the microstructured molding tool, backing or liner. Thus,
pressure-sensitive adhesives which flow and soften under the
embossing process conditions and, when cooled if heat is required
to impart a microstructured surface, maintain the microstructured
pattern are required. Particularly well suited for this use are
thermoplastic block copolymer adhesives, including but not limited
to those selected from the group consisting of
styrene-isoprene-styrene, styrene-butadiene-styrene, and
styrene-ethylene/butylene-styrene block copolymers, such as the
tackified adhesives described in U.S. Pat. Nos. 3,635,752 and
4,136,071 (Korpman) and U.S. Pat. Nos. 3,880,953 and 3,953,692
(Downey), all incorporated by reference herein. More preferably,
acrylic polymeric pressure-sensitive adhesives modified with
grafted high glass transition temperature (Tg) polymeric segments
(Tg higher than the application temperature, but considerably lower
than the processing temperature), such as those described in U.S.
Pat. No. 4,554,324 (Husman et al.), incorporated herein by
reference, can be embossed. Husman et al. describe a hot melt
processible acrylate PSA which gains the needed balance of cohesive
strength, high tack, and low melt viscosity through the chemical
modification of the soft acrylate backbone by adding or grafting
reinforcing high Tg polymeric moieties to the acrylate chain. These
high Tg pendant moieties provide a "physical crosslinking" by
forming glassy domains which enhance the cohesive strength of the
adhesive at lower temperatures without significantly increasing the
melt viscosity of the composition. Both of these classes of
pressure-sensitive adhesive exhibit thermoplastic behavior,
softening at processing temperatures and hardening when cooled, due
to the morphological properties or their high Tg segments. The high
Tg polymeric moieties typically have a Tg of above about 20.degree.
C. and a molecular weight in the range of about 2,000 to 30,000.
The glassy domains formed by these high Tg polymeric moieties
become fluid-like under process conditions and coalesce when cooled
to reform glassy domains which function as thermally reversible,
physical crosslinking sites.
The pressure-sensitive adhesive compositions useful in the
embossing process of this invention can be in the form of
solutions, emulsions or dispersions, or as hot melt coatings
depending on the end use and process methods or conditions utilized
in preparing the microstructured tapes and transfer coatings.
Preferably, solventless and/or hot melt coatable pressure-sensitive
compositions are employed. If a solventborne or waterborne
pressure-sensitive adhesive composition is employed in any of the
embossing methods, then the adhesive layer must undergo a drying
step to remove all or a majority of the carrier liquid prior to
embossing.
Microstructured pressure-sensitive adhesive tapes and transfer
coatings prepared by methods which involve the coating of the
adhesive on the microstructured molding tool, backing or liner may
employ a variety of pressure-sensitive adhesive formulations and
coating methods. The main limitation on this method is maintaining
a sufficiently low viscosity of the pressure-sensitive adhesive
such that the coating flows into the pattern of and easily
displaces the air in the microstructured molding tool, backing or
liner. Pressure-sensitive adhesives which can be hot-melt
processed, such as the tackified block copolymer adhesives and the
high Tg macromonomer modified acrylic polymeric pressure-sensitive
adhesives described above, can be directly extruded onto the
microstructured liner. If this coating method is used, then the
extrusion die temperature must be high enough to cause the glassy
domains of these adhesives to soften and allow the adhesives to
flow into the microstructured features of the liner. The adhesive
becomes cohesively strong as soon as the coating reaches a
temperature below that of the glass transition temperature of the
thermoplastic component of the adhesive.
Cast from concentrated solution, emulsion or dispersion, other
classes of pressure-sensitive adhesive compositions can also be
coated on the microstructured molding tools, backings and liners in
accordance with these methods. Examples of such pressure-sensitive
adhesive compositions include but are not limited to those selected
from the group consisting of organic solvent based acrylics,
waterborne acrylics, silicone adhesives, natural rubber based
adhesives, and thermoplastic resin based adhesives. When organic
solvent based or waterborne adhesive compositions are employed,
coating on the microstructured molding tool, backing or liner must
be followed by a drying step which is required to evaporate the
carrier liquid from the coating. In these cases, a somewhat
deformed pattern of the microstructured liner can be imparted to
the adhesive coating. Suitable carrier liquids are those which are
inert to the adhesive and to the liner and will not otherwise
adversely affect the coating and drying procedure. Examples of such
carrier liquids include but are not limited to those selected from
the group consisting of water and organic solvents such as ethyl
acetate, acetone, methyl ethyl ketone, and mixtures thereof.
Depending on the concentration of the adhesive solution, the
microstructured surface of the adhesive layer will differ from the
microstructure of the molding tool, backing or liner due to
shrinking upon drying. Thus, in order to minimize shrinkage of the
microstructured surface from adhesives cast from solution, emulsion
or dispersion, the concentration of the carrier liquid should be as
low as possible to result in an adhesive of a sufficiently low
viscosity to flow into the features of the microstructured
liner.
Finally, a prepolymerized, radiation curable acrylate
pressure-sensitive adhesive syrup containing a photoinitiator
having sufficient coatable viscosity to conform to the features of
the microstructured molding tool, liner or backing can be coated
onto the microstructured molding tool, backing or liner according
to these methods. While maintaining an oxygen free or near-oxygen
free atmosphere, such photopolymerizable syrups may be cured after
being coated on a microstructured molding tool, liner or backing by
irradiation with ultraviolet light, as described in U.S. Pat. No.
4,181,752 (Martens et al.), incorporated herein by reference. A key
requirement of microstructured pressure-sensitive adhesive articles
made in this manner is that, following the coating step, the
adhesive layer must be capable of exposure to an ultraviolet light
source. To meet this requirement, unless an exposed surface of
adhesive is present, at least one of the backings, liners or
microstructured molding tools employed in the microstructuring
process must allow the transmission of ultraviolet radiation to the
microstructured pressure-sensitive adhesive layer.
Rheological Properties of Pressure-Sensitive Adhesives
The peel characteristics of the microstructured adhesives of this
invention, aside from depending on the surface pattern and the
chemical nature of the pressure-sensitive adhesive formulation
used, can also be controlled through the modification of the
rheological properties of the adhesive. The degree of crosslinking
is one means for modifying pressure-sensitive adhesive rheology by
selectively controlling the long term flow of the
pressure-sensitive adhesive coating and the further wetting of
adherend. The microstructured pressure-sensitive adhesives of this
invention can be crosslinked by heat or radiation, forming
covalently crosslinked networks which modify the adhesive's flowing
capabilities. Alternatively, the physical crosslinking
characteristics of the thermoplastic tackified block copolymer and
the high Tg macromonomer modified acrylic polymeric
pressure-sensitive adhesives described above can be used. These
thermoplastic coatings can be further crosslinked by radiation,
preferably by exposure to electron beams or formulated according to
their proportion and/or molecular weight of high Tg polymeric
segments, relying on the glassy domains of these adhesives to
control the degree of long term adhesive flow.
Although in many applications a physical crosslinking of PSAs (by
means of a presence of coreacted thermoplastic component in the
polymeric system) is sufficient, the microstructured adhesive can
be subjected to various processes which would provide a permanent
(chemical) crosslinking to the PSA coatings.
Crosslinking agents can be added to all types of adhesive
formulations but, depending on the coating and processing
conditions, curing can be activated by thermal or radiation energy,
or by moisture. In cases in which crosslinker addition is
undesirable one can crosslink the microstructured adhesive if
needed by exposure to an electron beam.
The degree of crosslinking can be controlled to meet specific
performance requirements. For instance, for the PSA coatings in
which a low initial adhesion should be followed by build-up of
adhesion, no chemical crosslinking is needed and a low degree of
physical "crosslinking" would be required. For permanently
repositionable adhesives some degree of chemical crosslinking might
be desirable, but in order not to badly affect the tack of the
adhesive crosslink density has to be kept low. Tightly crosslinked
adhesives could have increased elastomeric character thus being
more prone to detach from a target substrate with time.
The PSA can optionally further comprise one or more additives.
Depending on the method of polymerization, the coating method, the
end use, etc., additives selected from the group consisting of
initiators, fillers, plasticizers, tackifiers, chain transfer
agents, fibrous reinforcing agents, woven and non-woven fabrics,
foaming agents, antioxidants, stabilizers, fire retardants,
viscosity enhancing agents, and mixtures thereof can be used.
Backings may be of any material which is conventionally utilized as
a tape backing or may be of other flexible material, the only
limitation being that the backing have adequate thermal stability
so not to be degraded or deformed by the heat embossing process
used in some of the methods of this invention. Such backings
include, but are not limited to those selected from the group
consisting of poly(propylene), poly(ethylene), poly(vinyl
chloride), polyester [e.g., poly(ethylene terephthalate)],
polyamide films such as dupont's Kapton.TM., cellulose acetate, and
ethyl cellulose. Backings may also be of woven fabric formed of
threads of synthetic or natural materials including but not limited
to those materials selected from the group consisting of cotton,
nylon, rayon, glass or ceramic material, or they may be of nonwoven
fabric such as air laid webs of natural or synthetic fibers or
blends of these. In addition, the backing may be formed of
materials selected from the group consisting of metal, metallized
polymeric film, and ceramic sheet materials.
The PSA compositions employed in the articles and methods of the
present invention can be coated onto backings without modification
by extrusion, coextrusion, or hot melt techniques, roll coating,
knife coating, curtain coating, and the like by employing
conventional coating devices for this purpose.
TEST METHODS
Peel Adhesion
[Reference: ASTM D3330-78 PSTC-1 (11/75)]
Peel adhesion is the force required to remove a coated flexible
sheet material from a test panel measured at a specific angle and
rate of removal. In the examples, this force is expressed in
Newtons per decimeter (N/dm) width of coated sheet. Following
equilibration of samples at 50% relative humidity and 22.degree.
C., both immediate and aged (24 hours dwell) peel adhesion
measurements were taken following the application of the sample to
a glass test surface. The procedure followed was:
1. A 12.7 mm width of the coated sheet was applied to the
horizontal surface of a clean glass plate with at least 12.7 lineal
cm in firm contact. A 2 kg hard rubber roller was used to apply the
strip.
2. The free end of the coated strip was doubled back nearly
touching itself so the angle of removal was 180.degree.. The free
end was attached to the adhesion tester scale.
3. The glass test plate was clamped in the jaws of a tensile
testing machine which was capable of moving the plate away from the
scale at a constant rate of 2.3 meters per minute.
4. The scale reading in Newtons was recorder as the tape was peeled
from the glass surface. The data is reported as the average of the
range of
numbers observed during the test.
Percent Wetout
This technique is used to study the wetting of a rough-surfaced
adhesive onto a smooth transparent substrate. The hardware used
with this technique consists of a stereo-microscope (Olympus Model
SZH-ZB), a video-camera (Cohu Model 4815) mounted on the
microscope, a coaxial vertical illuminator (Olympus Model TL2), and
a computer (Hewlett-Packard Vectra.TM. QS/20) with a video
digitizing board (Imaging Technologies PCVISIONplus.TM.) installed
which allows the computer to capture and digitize an image. Such an
image can subsequently be stored and analyzed by commercial
software packages (Jandel JAVA.TM.). The coaxial vertical
illuminator provides light which is sent through the lens (i.e.,
the optic axis) to illuminate the subject. This light passes
through a circular polarizer mounted on the end of the planar
objective lens of the microscope. In practice, the procedure is as
follows:
1. Apply the adhesive tape onto a glass (or other optically clear
and flat) surface in a reproducible manner.
2. Position the laminate so that the adhesive/glass interface is
viewed through the glass by a stereo microscope.
3. Adjust the sample so that the glass is perpendicular to the
optic axis.
4. Adjust the circular polarizer to optimize light intensity and
contrast.
5. Using the image analysis software, capture and digitize the
image.
6. Set the software grey value window of acceptance to accept only
those grey values (i.e., brightness levels) corresponding to the
wet areas.
7. Analyze the total wet area as a percentage of the total imaged
area.
This technique can be used both to monitor the adhesive samples'
wetout patterns over time and measure the percent wetout of a
single bonded sample with time. The percent wetout values shown in
the following table were derived from images taken at the same
location of adhesive/glass substrate at different times after
application.
______________________________________ Abbreviations and Trade
Names The following abbreviations and trade names are used herein:
______________________________________ IOA Isooctyl acrylate AA
Acrylic acid PS Macromer Polystyrene with terminal methacrylate
group ACMAS Acrylamidoamido Siloxanes MW Number average molecular
weight ______________________________________
PREPARATION OF MICROSTRUCTURED MOLDING TOOLS
EXAMPLE 1
Preparation of 10K ACMAS and 35K ACMAS Molding Material
A diamino functional polysiloxane terminated on both ends with
ethylenically unsaturated groups were prepared by the method
described in U.S. Pat. No. 5,091,483 (Mazurek et al.), incorporated
herein by reference and described below. A 500 M1 3-necked round
bottom flask equipped with thermometer, mechanical stirrer,
dropping funnel, and dry argon inlet was charged with 7.74 g of
bis(3-aminopropyl) tetramethyldisiloxane and 36 g of
octamethylcyclotetrasiloxane (D.sub.4) which had been previously
purged for 10 minutes with argon. The flask contents were heated to
80.degree. C. with an oil bath, following which a trace amount
(about 0.03 to 0.05 g) of an anhydrous 3-aminopropyl dimethyl
tetramethylammonium silanolate catalyst was added via a spatula to
the flask contents. The reaction mixture was stirred at 80.degree.
C. and after 30 minutes of stirring had become quite viscous. Vapor
phase chromatography (VPC) showed that the end-blocker had
completely disappeared. To the resultant reaction mixture (which
consisted of a 1,500 number average molecular weight polysiloxane
with aminopropyl end groups, cyclic siloxanes, and active catalyst)
was added dropwise over a six hour period 310 g of argon-purged
D.sub.4, resulting in a further rise in the viscosity. The reaction
flask contents were maintained at 80.degree. C. overnight. The
catalyst was decomposed by heating at 150.degree. C. for 1/2 hour,
and the product was stripped at 140.degree. C. at 0.1 mm pressure
until no more volatiles distilled (ca. 1/2 hours), resulting in 310
g of a clear, colorless, viscous oil (a yield of 88% of
theoretical). The number average molecular weight of the product
determined by acid titration was 10,000. Using this procedure, but
varying the ratio of endblocker to D.sub.4, a silicone diamine with
a number average molecular weight of 35,000 was also prepared.
A polydimethylsiloxane terminated on both ends with acrylamidoamido
groups (ACMAS) and having a number average molecular weight of
about 10,000 was prepared by thoroughly mixing 100 g (0.01 mole) of
aminopropyl-terminated polydimethylsiloxane prepared according to
the above description with 2.78 g (0.02 mole) vinyl dimethyl
azlactone (VDM), prepared as described in U.S. Pat. No. 4,777,276
(Rasmussen et al.), at room temperature. The viscosity of the
reaction mixture increased as the reaction progressed. The number
average molecular weight of the difunctional polysiloxane was
determined by acid titration of the precursor and was confirmed by
gel permeation chromatography (GPC) analysis before and after
capping with VDM. A 35,000 MW ACMAS was prepared similarly.
EXAMPLE 2
This example demonstrates the blending of the ACMAS materials of
Example 1 for the preparation of the microstructured molding tools
and the making of the microstructured molding tools used in
imparting microstructures to the pressure-sensitive adhesives of
the present invention. 10,000 MW ACMAS (5.0 g) was mixed with
35,000 MW ACMAS (5.0 g) and 0.02 g
2-hydroxy-2-methyl-1-phenyl-propan-1-one, available from EM
Industries under the tradename Darocur.TM. 1173 and was coated to a
thickness of 2 mm against a microstructured surface. After covering
the non-structured surface of the coating with a polyester film,
the mixture was then exposed to UV irradiation at 2.6 mW/cm.sup.2
(Sylvania Blacklight) for 10 minutes and the mold was separated
from the microstructured surface.
The following microstructured molding tools were made using the
following microstructured surfaces:
EXAMPLE 3
Convex Hemispheres (Positive Features)
A microstructured molding tool having convex hemispheres as
features on its surface was prepared in the following manner from a
glass microsphere embedded liner made in accordance with U.S. Pat.
No. 4,025,159 (McGrath), incorporated herein by reference: Glass
microspheres ranging between about 50 and 80 .mu.m were embedded by
standard procedures to about 40 percent of their diameter in a 25
.mu.m-thick layer of polyethylene which is carried on paper. The
glass microspheres are released from the polyethylene, leaving an
aperiodic concave hemispherical microstructured polyethylene liner
as shown in top plan view in FIG. 6. The polyethylene liner was
then attached to a glass plate and the glass plate bordered by a
gasket. The microstructured molding tool having a convex (i.e.,
positive) microstructure was made by coating and curing a layer of
liquid silicone rubber as described in Example 2 against the
polyethylene liner and separating the cured silicone
microstructured molding tool from the polyethylene liner. The
microstructured molding tool had positive features of an average
feature height of approximately 50 .mu.m, lateral aspect ratio
(LAR) of approximately 2.5, and a spacing aspect ratio (SAR) of
approximately 1.5.
EXAMPLE 4
Cube Corners of Positive Features
A microstructured master was prepared by micromachining a cube
corner (i.e., triangular pyramid) pattern having positive features
into a metal plate in accordance with the methods described in U.S.
Pat. No. 4,558,258 (Hoopman), incorporated herein by reference, and
illustrated in top plan view by FIG. 7. Using standard techniques,
a nickel electroform replica (i.e., having negative features) of
the microstructured master was then formed. A microstructured
molding tool with cube corners of positive features on its surface
was made by coating and curing a layer of the liquid silicone
rubber as described in Example 2 onto the microstructured nickel
electroform replica and separating the cured microstructured
molding tool from the electroformed replica. The resulting
microstructured molding tool had positive features (as illustrated
in cross-sectional view in FIG. 8) of an average feature height of
approximately 62.5 .mu.m, lateral aspect ratio (LAR) of
approximately 2, and a spacing aspect ratio (SAR) of approximately
1.
EXAMPLE 5
Cube Corners of Negative Features
A microstructured molding tool with cube corners of negative
features on its surface was made by coating and curing a layer of
the liquid silicone rubber as described in Example 2 onto a
positive-featured second nickel electroformed replica formed by
replicating the nickel electroform of Example 4. Following the
separation of the cured silicone materials from the second nickel
electroform, a microstructured molding tool having negative
features (as illustrated in cross-sectional view in FIG. 9) of an
average feature depth of approximately 62.5 .mu.m, lateral aspect
ratio (LAR) of approximately 2, and a spacing aspect ratio (SAR) of
approximately 1 was formed.
The following microstructured liners were made and used in the
preparation of microstructured PSAs.
EXAMPLE 6
V-Shaped Grooves
A roll of microstructured liner, which was also used as a
microstructured molding tool, was made in a continuous process by
the following method: A microstructured roll having V-shaped
grooves (feature height=50 .mu.m, lateral aspect ratio (LAR)=2,
spacing aspect ratio (SAR)=1) was prepared using a micromachining
tool in a machinable substrate. The liquid silicone rubber of
Example 2 was then coated against an unprimed polyester film and
the silicone rubber was simultaneously embossed by the
microstructured roll and cured by exposure to high intensity UV
light. This process resulted in a microstructured liner which
replicated the features of the microstructured roll.
EXAMPLE 7
Concave Hemispheres
A microstructured liner with negative hemispherical features on its
surface was made by depositing a UV curable epoxysilicone release
layer on the microstructured surface of the concave microstructured
polyethylene liner described in Example 3. The microstructured
liner had negative features of an average feature depth of
approximately 50 .mu.m, lateral aspect ratio (LAR) of approximately
2.5, and a spacing aspect ratio (SAR) of approximately 1.5.
PREPARATION OF ADHESIVE COMPOSITIONS
EXAMPLE 8
Preparation of Polystyrene Macromer
A methacrylate-terminated polystyrene polymeric monomer having an
average molecular weight of 10,000 was prepared in accordance with
U.S. Pat. No. 4,554,324 (Husman et al.), incorporated herein by
reference, and described below. A flame-dried 5 liter glass
5-necked flask equipped with a mechanical stirrer, gas inlet,
condenser, addition funnel, and thermometer was purged with dry
argon, and charged with 2100 g cyclohexane which had previously
been distilled from polystyryl lithium. The cyclohexane was heated
to 50.degree. C. and 20 ml of a 1.17 molar solution of
sec-butyllithium in cyclohexane (23.4 millimoles) were added to the
flask via a syringe. Purified styrene monomer (175 g) was added in
one portion to the flask, resulting in an exothermic reaction. The
temperature was maintained at less than 74.degree. C. by cooling
and then, during the next hour, the reaction mixture was maintained
at approximately 50.degree. C. Thereafter, the mixture was cooled
to 40.degree. C. and ethylene oxide previously passed over sodium
hydroxide was introduced with vigorous stirring until the red color
of polystyryl lithium had changed to a faint yellow. Thereupon the
reaction was quenched with 1.4 g (23.4 millimoles) acetic acid. The
reaction mixture was saturated with dry air, 10.9 g (70.2
millimoles) 2-isocyanatoethyl methacrylate and 4 drops of tin
dioctoate catalyst were added, and the resultant mixture was heated
to 60.degree. C. and maintained at that temperature for 14
hours.
The mixture was then cooled and the polymer was precipitated in 30
liters of methanol, dried in vacuo, to yield 170 g (97% yield)
methacrylate-terminated polystyrene monomer having a number average
molecular weight of 9,600, a weight average molecular weight of
10,464, and a polydispersity of 1.09 as determined by conventional
gel permeation chromatography (GPC).
EXAMPLE 9
Preparation of "Hard" Pressure-Sensitive Adhesive
A thermoplastic pressure-sensitive adhesive having a high
concentration of high glass transition temperature (Tg) grafted
polymeric segments (i.e., a "hard" pressure-sensitive adhesive)
consisting of a copolymer of 84 parts by weight isooctyl acrylate,
6 parts by weight acrylic acid, and 10 parts by weight the
methacrylate-terminated polystyrene polymeric monomer of Example 8
was prepared as described in U.S. Pat. No. 4,554,324 (Husman et
al.), incorporated herein by reference.
In a glass reaction bottle was placed 14.8 grams isooctyl acrylate,
1.2 gram acrylic acid, 2.0 grams methacrylate-terminated
polystyrene macromer of Example 3, 2.0 grams of a stock solution
consisting of 0.5 g carbon tetrabromide and 99.5 g isooctyl
acrylate, 30 grams of ethyl acetate and 0.06 grams of
2,2'-azobis(isobutyronitrile) free radical initiator available
under the trademark "VAZO" 64 from the E. I. DuPont Company. The
reaction bottle was purged with nitrogen, sealed, and tumbled in a
55.degree. C. water bath for 24 hours.
EXAMPLE 10
Preparation of "Soft" Pressure-Sensitive Adhesive
A thermoplastic pressure sensitive adhesive having a low
concentration of high Tg grafted polymeric segments (i.e., a "soft"
pressure-sensitive adhesive) consisting of a copolymer of 92 parts
by weight isooctyl acrylate, 4 parts by weight acrylic acid, and 4
parts by weight the methacrylate-terminated polystyrene polymeric
monomer of Example 8 was prepared in the same manner as Example
9.
EXAMPLE 11
Preparation of Tackified Block Copolymeric Pressure-Sensitive
Adhesive
A thermoplastic pressure-sensitive adhesive was prepared by mixing,
in sufficient toluene to form a 40% by weight solids solution, 60
parts by weight of a styrene-isoprene-styrene (ABA) block copolymer
(Kraton.TM. D1107, commercially available from Shell Chemical Co.)
and 40 parts by weight of a solid tackifying resin (Wingtack
Plus.TM., an aromatically modified petroleum resin having a ring
& ball softening point of 93-100.degree. C., a specific gravity
of 0.93 at 25.degree. C., and a molecular weight of 1100 available
from Goodyear Tire and Rubber Co.).
EXAMPLE 12
Preparation of a Waterborne Acrylic Pressure-Sensitive Adhesive
A waterborne acrylic pressure-sensitive adhesive was prepared
according to the emulsion polymerization method of Example 5 of
U.S. Pat. No. Re. 24,906 (Ulrich), incorporated herein by
reference. A mixture of 104 parts by weight distilled water, 8
parts by weight of a 28% solution of alkylated aryl polyether
sodium sulfonate (commercially available as Triton X-200.TM. from
Union Carbide Chemicals and Plastics Co.), 95 parts isooctyl
acrylate, 5 parts acrylic acid, and 0.08 parts tertiary dodecyl
mercaptan was purged with nitrogen and brought to 30.degree. C.
with agitation. There was then added 0.2 part of potassium
persulfate and 0.067 part of sodium bisulfate. Following
polymerization, the acrylic polymer (having an inherent viscosity
of 1.05 to 1.35 in tetrahydrofuran) was recovered from emulsion and
dissolved in an 80/20 mixture of heptane and propyl alcohol to a
solids content of 44.2% by weight.
EXAMPLE 13
Preparation of a UV Curable Pressure-Sensitive Adhesive
A radiation curable pressure-sensitive adhesive used to prepare
microstructured PSA tapes by radiation curing against a
microstructured molding tool, liner and/or backing was prepared in
the following manner: A
mixture of 90 parts isooctyl acrylate, 10 parts of acrylic acid,
0.04 part 2,2-dimethoxy-2-phenyl acetophenone (obtained as
Escacure.TM.-KB-1 from Sartomer) was inerted and partially
photopolymerized to a conversion of about 7% under ultraviolet (UV)
irradiation (40 watt fluorescent black lamp having 90% of the
emissions between 300 and 400 nm and a maximum at 351 nm and which
provides radiation intensity of about 1-2 mW/cm.sup.2) to yield a
coatable syrup of about 3,000 cPs. Prior to coating on a
microstructured molding tool, liner or backing, 0.1 part of
Escacure.TM.-KB1 and 0.1 part 1,6-hexanediol diacrylate (HDDA) were
added to the syrup with thorough mixing.
PREPARATION AND PELL ADHESION TESTING OF EMBOSSED ADHESIVE
SAMPLES
EXAMPLE 14
A continuous layer of the "hard" thermoplastic adhesive of Example
9 was coated onto a 37.5 .mu.m (0.0015 inch) primed polyester film
using knife coater to a thickness of 62.5 .mu.m (0.0025 inch) and
the coating was dried in the oven at 60.degree. C. for 10 min. The
coated adhesive was embossed using the cube corners microstructured
silicone molding tool of Example 5 (cube corners of negative
features) using a Carver Laboratory Press Model M at 120.degree. C.
and 15,000 psi pressure for 30 minutes. During embossing the
adhesive coating in contact with the silicone molding tool was
placed between the two metal plates, cushioned by a layer of the
heat resistant foam and surrounded by a aluminum spacer to prevent
an excessive pressure. After cooling, the microstructured
pressure-sensitive adhesive tape sample was separated from the
molding tool, cut into 25 mm wide strips, and subjected to both
immediate and aged 180.degree. peel adhesion testing as described
above against a glass target substrate. The results of these tests
are recorded in Table 1.
EXAMPLE 15
A microstructured pressure sensitive adhesive tape sample was
prepared and tested in accordance with Example 14, except that the
tape sample was coated with the "soft" thermoplastic
pressure-sensitive adhesive of Example 10. The results of these
tests are recorded in Table 1.
EXAMPLE 16
A microstructured pressure-sensitive adhesive tape sample was
prepared and tested in accordance with Example 14, except that the
tape sample was embossed with the microstructured molding tool of
Example 4 (cube corners of positive features). The results of these
tests are recorded in Table 1.
EXAMPLE 17
A microstructured pressure-sensitive adhesive tape sample was
prepared and tested in accordance with Example 16, except that the
tape sample was coated with the "soft" thermoplastic
pressure-sensitive adhesive of Example 10. The results of these
tests are recorded in Table 1.
COMPARATIVE EXAMPLE C-1
A comparative pressure-sensitive adhesive tape sample was prepared
by coating a continuous layer of the thermoplastic adhesive of
Example 9 using a knife coater to a dry thickness of 62.5 .mu.m
(0.0025 inch) onto a primed polyester 37.5 .mu.m (0.0015 inch). The
coated planar adhesive was dried in an oven for 10 minutes at
60.degree. C., cooled, cut into 25 mm wide strips, and subjected to
both immediate and aged 180.degree. peel adhesion testing as
described above against a glass target substrate. The results of
these tests are recorded in Table 1 and Table 2.
COMPARATIVE EXAMPLE C-2
A comparative pressure-sensitive adhesive tape sample was prepared
and tested in accordance with Comparative Example C-1, except that
the tape sample was coated with the "soft" thermoplastic
pressure-sensitive adhesive of Example 10. The results of these
tests are recorded in Table 1 and Table 2.
TABLE 1 ______________________________________ Cube Corners
Adhesive Peel Adhesion (N/dm) Example (+/-) (hard/soft) Immediate
24 hours ______________________________________ 14 + hard 43.8 4.4
15 + soft 48.2 16 - hard 4.4 17 - soft 15.3 C-1 hard 81.0 C-2 soft
63.5 ______________________________________ "+" denotes cube
corners of positive features "-" denotes cube corners of negative
features "hard" denotes the 84/6/10 IOA/AA/PS Macromer
thermoplastic pressuresensitive adhesive of Example 9 "soft"
denotes the 92/4/4 IOA/AA/PS Macromer thermoplastic
pressuresensitive adhesive of Example 10
Analysis of the data presented in Table 1 indicates that in
microstructured pressure-sensitive adhesives two main forces can
counteract: adhesion to the substrate and a restorative elastomeric
forces of the adhesive. Harder, more elastomeric adhesives having a
small contact area with the target substrate (e.g. Example 16) tend
to be self-debonding, showing a decrease of peel force with time.
On the other hand, the peel force of softer adhesives having the
same microstructured pattern demonstrates permanent
repositionability in Example 17.
More dramatic changes are observed in the case of adhesives with
positive features (i.e., cube corners out). The soft adhesive of
Example 15 demonstrates permanent repositionability but, due to a
higher proportion of contact area with the target substrate, at a
higher level of initial and aged peel adhesion than Example 17. The
hard adhesive of Example 14 embossed with the same structure shows
a strong initial adhesion which drops with time; after 24 hours it
is very low. Thus, through controlling both the rheology and the
microstructured surface pattern the pressure-sensitive adhesives of
the present invention, adhesive coatings having a range of initial
repositionabilities and aged adhesion characteristics can be
obtained.
EXAMPLES 18 AND 19
These examples illustrate that permanently repositionable
microstructured pressure-sensitive adhesive tapes which perform in
different peel adhesion ranges can be made by varying the adhesive
composition on identical microstructures. In Example 18, a
microstructured pressure sensitive adhesive tape sample was
prepared and tested in accordance with Example 14, except that the
microstructured molding tool of Example 3 (positive hemispheres)
was used. Example 19 was prepared in accordance with example 18
except that the "soft" adhesive of Example 10 was used. The results
of immediate and aged peel testing from glass of these samples
having convex hemispheric features are recorded in Table 2. Percent
wetout was also measured and recorded at different time intervals
after being applied to a glass test surface using light application
force (0.85 Newtons) and heavier force (20 Newtons) in Table 3.
EXAMPLE 20
A microstructured pressure-sensitive adhesive tape sample was
prepared in accordance with example 14, except that the tape sample
was coated with tackified block copolymeric thermoplastic adhesive
of Example 11 and was embossed with the microstructured molding
tool of Example 6 (microstructured liner) with a V-groove
microstructured surface. Both the initial adhesion and aged
180.degree. peel adhesion to glass target substrate were measured
and recorded in Table 2.
COMPARATIVE EXAMPLE C-3
A comparative pressure-sensitive adhesive tape was prepared in
accordance with Comparative Example C-1, except that the tape
sample was coated with the thermoplastic adhesive of Example 11 to
a dry thickness of 50 .mu.m (0.002 inch). Both the initial adhesion
and aged 180.degree. peel adhesion to glass target substrate were
measured and recorded in Table 2.
EXAMPLE 21
The following example illustrates a microstructured pressure
sensitive adhesive tape which builds adhesion with time. A
microstructured pressure-sensitive adhesive tape was prepared by
casting using a knife coater the waterborne pressure-sensitive
adhesive of Example 12 against the microstructured liner prepared
as in Example 6 to a dry thickness of 50 .mu.m (0.002 inch). After
drying the sample in the oven at 60.degree. C. for 10 minutes, the
adhesive was laminated to a primed polyester film using GBC Desktop
laminator. The pressure-sensitive adhesive tape sample was
subsequently removed from the microstructured liner and applied to
a glass target substrate. The results of immediate and aged peel
testing from glass are recorded in Table 2. Percent wetout was also
measured and recorded in Table 3 at different time intervals after
being applied to a glass test surface using light application force
(0.85 Newton).
COMPARATIVE EXAMPLE C-4
A comparative pressure-sensitive adhesive tape was prepared in
accordance with Comparative Example C-1, except that the tape
sample was coated with the waterborne adhesive of Example 12 to a
dry thickness of 50 .mu.m (0.002 inch). Both the initial adhesion
and aged 180.degree. peel adhesion to glass target substrate were
measured and recorded in Table 2.
EXAMPLE 22
A microstructured pressure-sensitive adhesive tape was prepared by
coating a radiation curable adhesive of Example 13 between the
microstructured liner of Example 7 and primed polyester film,
exposing the construction to low-intensity UV lights through the
film for 5 min. The pressure-sensitive adhesive tape sample was
subsequently removed from the microstructured liner and applied to
a glass target substrate. Both the initial and aged 180.degree.
peel adhesion force was measured and recorded in Table 2. Percent
wetout was also measured and recorded in Table 3 at different time
intervals after being applied to a glass test surface using light
application force (0.85 Newton).
COMPARATIVE EXAMPLE C-5
A comparative pressure-sensitive adhesive tape was prepared and
tested in accordance with comparative Example C-1, except that the
tape sample was coated with radiation-curable adhesive of Example
13 to a thickness of 100 .mu.m (0.004 inch). Both the initial and
aged 180.degree. peel adhesion force was measured and recorded in
Table 2.
EXAMPLE 23
The following example illustrates a method of making a
microstructured pressure-sensitive adhesive tape by applying
compressive forces at room temperature to an embossable adhesive in
contact with a microstructured surface. A microstructured
pressure-sensitive adhesive tape was prepared by coating a layer of
the waterborne pressure-sensitive adhesive of Example 12 with a
knife-coater to a thickness of 37.5 .mu.m (0.0015 inch) on a
polyester backing, drying the unstructured tape in an oven, and
applying to the V-grooved microstructured liner of Example 6 to the
exposed surface of the adhesive layer. To the top of this 5
cm.times.5 cm laminate a dead-load of 73.5 Newtons was placed for
16 hours to emboss the adhesive surface under this compressive
force at room temperature. Both the initial adhesion and aged
180.degree. peel adhesion to glass target substrate were measured
and recorded in Table 2.
TABLE 2 ______________________________________ Pressure- Sensitive
Micro- Adhesive Peel Adhesion (N/dm) Ex. structure (Example No.)
Immediate 24 hours % change 18 Hemisphere Hard 11.4 8.3 -27% in
Thermoplastic (9) C-1 Planar Hard 61.3 81.0 +32% Thermoplastic (9)
19 Hemisphere Soft 22.8 19.7 -14% in Thermoplastic (10) C-2 Planar
Soft 74.4 63.5 -15% Thermoplastic (10) 20 V-Groove Block 33.3 52.1
+56% Copolymer (11) C-3 Planar Block 106.4 114.3 +7% Copolymer (11)
21 V-Groove Waterborne 18 41.6 +131% (12) C-4 Planar Waterborne
35.5 58.9 +66% (12) 22 Hemisphere 129.4 139.4 +8% out UV Curable
(13) C-5 Planar UV Curable 122.6 170.1 +39% (13) 23 V-Groove
Waterborne 23.0 30.6 +33% (12)
______________________________________
TABLE 3 ______________________________________ Percent Wetout Ex-
Reading Reading Reading am- Reading 1 2 (min- 3 (min- 4 (min-
Reading 5 Reading 6 ple (initial) utes) utes) utes) (minutes)
(minutes) ______________________________________ 18* 35.4% 34.0%
37.3% 30.5% 34.1% ******** (105) (30) (310) (1240) 19* 38.0% 43.5%
46.1% 47.8% 51.6% ******** (105) (30) (310) (1240) 18** 58.3% 59.8%
59.8% 58.3% 50.0% ******** (120) (30) (455) (1440) 19** 75.3% 71.9%
74.8% 72.2% 69.1% ******** (120) (30) (455) (1440) 21* 43.0% 55.3%
63.8% 68.4% 72.3% 83.9 (30) (90) (210) (450) (1440) 22* 38.4% 35.5%
36.1% 33.5% ******** ******** (315) (120) (1380)
______________________________________ "*" denotes percent wetout
testing using a low application force (0.85 Newtons) "**" denotes
percent wetout testing using a high application force (20
Newtons)
In Example 18 (negative hemispheres with a hard adhesive) the peel
force decreases with time. The planar adhesive analog, Comparative
Example C-1, shows an increase of adhesion with time. The data of
Table 3 show that for this example both the high and low force
rolldown samples demonstrate a slight decrease in percent wetout
with time. Thus the elastic recovery force which is itself a
product of the microstructure shape, the higher modulus of the
adhesive and trapped air within the concave hemispheres dominates
the other effects in this example.
In Example 19 (negative hemispheres with a soft adhesive) the peel
force also decreases with time. The planar adhesive analog,
Comparative Example C-2, shows a decrease in peel adhesion with
time also. The data of Table 3 show that for this example the high
force rolldown sample shows a decrease in percent wetout with time.
The low force sample shows an increase in percent wetout with time,
but the percent wetout value obtained after one day is less than
either the initial or final wetout of the high force application.
Although not wishing to be bound by theory, it is believed that an
equilibrium percent wetout exists somewhere between 52% and 69% for
this adhesive/geometry combination applied to glass. Although this
is a softer adhesive, once again the microstructure shape, modulus,
and trapped air work to restore an equilibrium different from the
initial conditions.
In Example 20 (V-grooves with a block copolymer adhesive) the peel
force increases substantially with time. The planar adhesive of
Comparative Example C-3 shows a less dramatic increase of adhesion
with time.
In Example 21 (V-grooves with a waterborne adhesive) the peel force
also increases substantially with time. The planar adhesive analog,
Comparative Example C-4, also shows a substantial increase of
adhesion with time. The data of Table 3 show that for this example
the low force rolldown sample shows a substantial increase in
percent wetout with time. In this system it appears that the
chemical affinity, the nanoscale wetting and the microscale wetting
all work to increase the percent wetout with time. Trapped air is
not a factor here due to the grooved microstructure which allows
trapped air to escape from the interface between the adhesive
surface and the target substrate.
In Example 22 (V-grooves with a UV-cured adhesive) the peel force
increases slightly with time. The planar adhesive analog,
Comparative Example C-5, shows a substantial increase of adhesion
with time. The data of Table 3 show that for this example the low
force rolldown sample shows a slight decrease in percent wetout
with time. We may conclude with this formulation that the light
crosslinking of the adhesive by UV radiation tends to increase the
restoring force; this works to decrease the wetout with time.
However this effect is dominated by the chemical affinity and/or
nanoscale flow effects which tend to increase adhesion as witnessed
by the planar control. Once again, in this example trapped air is
not a factor.
In Example 23 (V-grooves with a waterborne adhesive), the peel
force increases with time, but not to the extent of the planar
analog, Comparative Example C-4. The similarly formulated and
microstructured tape sample of Example 21, which was made by an
alternative method, demonstrates an even greater increase of peel
adhesion over time when compared to this example.
Various modifications and alterations of this invention will become
apparent to those skilled in the art without departing from the
scope and spirit of this invention, and it should be understood
that this invention is not to to be unduly limited to illustrative
embodiments set forth herein.
* * * * *